How to log into Oracle Social as Supplier Portal User

Summary:

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I have created a supplier user. When I log into as Supplier Portal user and navigate to Social, systems asks to log into OSN:

However, as business user (internal user), I am able to log into oracle social automatically and start conversation with internal users. But for supplier users, when I try to create a conversation, system does not show the user name. Alternatively, if I try inviting with email address, systems says "User Was not found in expected external realm in oracle fusion".

Can someone help me how supplier portal user can log into social? Requirement is to have conversation between buyer and supplier through messaging for quicker updates.